Unfortunately insufficient rainproof

With us the conch stands as sandbox on the non-covered terrace. Although we close the shell always carefully, make sure that the edges are perfectly and additionally the cover with a stone complain, so that it can not slip even in strong winds, we have unfortunately after every heavy rain the same picture. The sandbox is completely flooded, the shell partially filled halfway with water. Even after pouring the water is the sand for days soaking wet yet depending on weather conditions.
This is not waterproof, the shell, I knew, but I did not expect, after a violent rainfall to be found the same picture, as if I would leave out the cover.
Therefore only 3 stars.